1. Field of the Invention
The invention relates to an erasing and cleaning apparatus, also called an erasing station, for cleaning cylindrical surfaces, in particular of printing form and blanket cylinders within printing presses with the aid of a cleaning cloth which can be moved by cleaning cloth transport means and which can be advanced intermittently.
2. Description of Prior Art
A cleaning system for printing forms is known, for example, U.S. Pat. No. 5,727,470 which discloses a method and an apparatus for controlling the washing operation in a printing press. The object to be achieved in that publication is based on the fact that it is necessary to make the control system for advancing the cleaning cloth more precise, more reliable and more flexible. This object is essentially achieved in that publication in that the cleaning cloth transport system receives pulses which are proportional to the travel from a clocked gear wheel via a sensor, which pulses are used for controlling the drive motors for the transport means of the cleaning cloth, in order for it to be possible to keep the respective transport paths of the cleaning cloth approximately constant.
In order to achieve defined transport paths, the transport path of the cleaning cloth is detected by the sensor device in the forward and backward direction. An exchangeable unit known as a washing slide-in unit has a sensing device in the form of a clocking shaft, on which the cleaning cloth rolls, and a sensor which is fastened to the printing press and senses a toothed clocking wheel connected to the clocking shaft, the thickness of the clean cloth roll also being sensed by a resiliently pressed-on lug which interacts mechanically with a second sensor on the press. A measure of the transport speed of the cleaning cloth which is simple to form is the frequency of the signals which is obtained from regular markings.
The abovementioned known arrangement requires a separate control mechanism which necessitates material expenditure and additionally considerable outlay on maintenance.
Furthermore, U.S. Pat. No. 6,694,879 discloses a generic cleaning apparatus for cleaning the surface of a blanket cylinder or any desired cylinder of a printing unit, having an intermittently operated drive for the continuous cleaning cloth. The transport means for the cleaning cloth likewise essentially comprise a clean cloth roll and a dirty cloth roll for receiving the cleaning cloth which is impregnated with wash fluid and is pulled off from the clean cloth roll to the dirty cloth roll. Here, the cleaning cloth is always held under tensile stress by pressure means in the form of a wash roll.
The cleaning apparatus is configured, in particular, as a slide-in system and, in the installed state, is assigned to the cylindrical body which is to be cleaned. The cleaning apparatus is brought into and out of contact with the cylinder by a positioning unit. A washing fluid supply system fixed to the machine is assigned to the cleaning apparatus. The clean cloth roll, the wash roll and the dirty cloth roll are mounted rotatably in two side parts arranged in parallel with one another. The wash roll has a shaft which has an elastic coating and serves as a pressure element in order to press the cleaning cloth against the cylindrical body and simultaneously guide the cleaning cloth. The side parts of the slide-in system are connected to one another via a crossmember. The cleaning cloth is guided in as large a wrapping angle as possible from the clean cloth roll over the wash roll in the pulling direction of the dirty cloth roll. The cleaning cloth is wetted with a washing fluid. Drive is imparted to the dirty cloth roll via a shaft and gear wheel. The dirty cloth roll is operated intermittently and pulls the cleaning cloth from the clean cloth roll over the wash roll step by step, which brings the cleaning cloth into contact with the cylindrical body to be cleaned, in the pulling direction and accommodates the cleaning cloth on a shaft. The intermittently operated drive has up to now been imparted to the dirty cloth roll in a conventional manner via an (electric) motor or a pneumatic or hydraulic drive element, such as a compressed air cylinder, a drive shaft or rod and a gear wheel chain.
One disadvantage of the previous solutions of a mechanical intermittently operated drive is that the duration of a work cycle, which results in the respective transport path for the cleaning cloth, always remains constant and no consideration is thus taken of the occurrence of a transport path difference for the intermittent cloth advancing operation as a function of the decrease in the cloth supply on the clean cloth roll or of the increase in the winding radius on the dirty cloth roll. The cycle duration (of the work cycle) or thee cycle rate should be at its greatest when the clean cloth roll is full (maximum cloth supply), in order to wind up a certain cloth length with a small winding radius of the dirty cloth roll, and should become smaller and smaller as the winding radii of the dirty cloth roll become greater, in order to maintain the certain cloth length, that is to say a constant transport path, with the result that a transport path difference which occurs given a constant cycle duration on account of the varying winding radius could be compensated for, or in order to keep the advancing of the cloth (transport path of the cloth) constant over the entire use period (the time it takes to convey the cleaning cloth from the clean cloth roll to the dirty cloth roll, that is to say until the cloth supply is exhausted).
Furthermore, the cleaning cloth always has to be held under tensile stress for known reasons. For this purpose, various braking devices for the shaft of the clean cloth roll, such as spring-actuated brakes, which counteract the rotational direction of the clean cloth roll when the cloth is advanced are already known (see. e.g., U.S. Pat. No. 5,377,591). Consideration is once again not taken here of the fact that the pulling force on the cleaning cloth decreases in the case of a driven dirty cloth roll when the winding radii on the dirty cloth roll become greater, and the braking action on the clean cloth roll which counteracts the rotational direction then has to become smaller when the winding radii on the dirty cloth roll become greater in order not to bring about ever increasing cloth tension or even standstill of the cloth advancing if the braking action is greater than the pulling force.